Super-Atmospheric Pressure Ion Sources: Application and Coupling to API Mass Spectrometer

Abstract

Pressurizing the ionization source to gas pressure greater than atmospheric pressure is a new tactic aimed at further improving the performance of atmospheric pressure ionization (API) sources. In principle, all API sources, such as ESI, APCI and AP-MALDI, can be operated at pressure higher than 1 atm if suitable vacuum interface is available. e gas pressure in the ion source can have di erent role for di erent ionization. For example, in the case of ESI, stable electrospray could be sustained for high surface tension liquid (e.g., pure water) under super-atmospheric pressure, owing to the absence of electric discharge. Even for nanoESI, which is known to work well with aqueous solution, its stability and sensitivity were found to be enhanced, particularly in the negative mode when the ion source was pressurized. For the gas phase ionization like APCI, measurement of gaseous compound also showed an increase in ion intensity with the ion source pressure until an optimum pressure at around 4-5 atm. e enhancement was due to the increased collision frequency among reactant ion and analyte that promoted the ion/molecule reaction and a higher intake rate of gas to the mass spectrometer. Because the design of vacuum interface for API instrument is based on the upstream pressure of 1 atm, some coupling aspects need to be considered when connecting the high pressure ion source to the mass spectrometer. Several coupling strategies are discussed in this paper.

Super‐atmospheric pressure ionization mass spectrometry and its application to ultrafast online protein digestion analysis

“…With the addition of a booster pump, an ion transport tube with an inner diameter of 0.5 mm could be used to improve the ion transmission efficiency while maintaining the pressure of the mass spectrometer at an operational value (Fig. (c)) . A Roots booster pump (PMB 003C, ULVAC, Chigasaki, Japan) was added between the main pumping port of the mass spectrometer and the original rotary vane pumps.…”

“…The API ion source can be sealed such that the production of precursor ions/charged species and the subsequent ionization process take place under a super‐atmospheric condition ( P > 1 atm). Super‐atmospheric pressure ionization MS has been implemented with electrospray, corona discharge ionization and field desorption by our group and with photoionization by Peng et al High‐pressure ionization using radioactive isotope and photoelectron attachment had also been performed for ion mobility mass spectrometry. These ion sources will be briefly discussed here.…”

“…We have fabricated a number of HP ion sources that could be easily attached onto a commercial mass spectrometer without significant modification of the original ion inlet interface. The main chamber of the ion sources can be of various sizes depending on how the ionization is implemented and the maximum operating pressure . For a discharge‐based chemical ionization source, it was relatively small because the optical inspection on the needle was not crucial.…”

“…Changing the i.d. of the ion transport tube from 0.25 to 0.5 mm enhanced the ion signal by a factor of more than 5 for both positive and negative ion modes under optimized pressure …”

High-Temperature Liquid Chromatography and the Hyphenation with Mass Spectrometry Using High-Pressure Electrospray Ionization

“…e details are discussed in our previous reports. [73][74][75] In the recent experiment with HTLC, a homemade ion transfer tube with 0.25 mm inner diameter was used to sample the generated ions inside the high-pressure vessel. 66)…”